A vane-type variable valve timing adjusting mechanism is, as shown in JP2001-159330A (U.S. Pat. No. 6,330,870B1), adapted in such a manner that a housing rotating in a timed relation to a crank shaft of an engine is disposed coaxially with a vane rotor connected to a cam shaft of an intake valve (or exhaust valve) and a plurality of vane-accommodating chambers formed in the housing respectively are divided into an advance hydraulic chamber and a retard hydraulic chamber by vanes (blade portions) at the outer periphery of the vane rotor. In addition, the hydraulic pressure in each hydraulic chamber is designed to be controlled by a hydraulic control valve to rotate the vane rotor relative to the housing, so that a displacement angle of the camshaft (cam shaft phase) to the crankshaft is varied to variably control valve timing.
In such vane-type variable valve timing adjusting mechanism, at the time of opening/closing the intake valve or the exhaust valve during engine operating, fluctuations of torque which the camshaft receives from the intake valve or the exhaust valve are transmitted to the vane rotor. In consequence, torque fluctuations in the retard direction or in the advance direction are exerted on the vane rotor. Thereby, when the vane rotor is subjected to torque fluctuations in the retard direction, the operating oil in the advance hydraulic chamber is to be subjected to such pressure as to be pushed out of the advance hydraulic chamber or when the vane rotor is subjected to torque fluctuations in the advance direction, the operating oil in the retard hydraulic chamber is to be subjected to such pressure as to be pushed out of the retard hydraulic chamber. In consequence, in a low-rotation region where pressures supplied from a hydraulic supply source are low, even when a displacement angle of the cam shaft is designed to be advanced by supplying the hydraulic pressure to the advance hydraulic chamber, the vane rotor is, as shown in a dotted line of FIG. 3, pushed back in the retard direction due to the torque fluctuations. As a result, the response time to a target displacement angle of the vane rotor is longer.
In order to solve this problem, as shown in JP2003-106115A (U.S. Pat. No. 6,763,791 B2), a one-way valve is disposed in each of a hydraulic supply passage of an advance hydraulic chamber and a hydraulic supply passage of a retard hydraulic chamber for preventing reverse flow of operating oil from the advance hydraulic chamber or the retard hydraulic chamber. Thereby, as shown in a solid line of FIG. 3, it is considered that this one-way valve is adapted to prevent the vane rotor from being pushed back in the reverse direction to the direction of a target displacement angle during variable valve timing controlling, improving response characteristic of the variable valve timing control.
In the variable valve timing adjusting mechanism, the one-way valve is disposed in each of the hydraulic supply passage of the advance hydraulic chamber and the hydraulic supply passage of the retard hydraulic chamber (hydraulic introduction line) and also a returning line (hydraulic discharge line) is disposed in parallel to the hydraulic supply passage of each hydraulic chamber for bypassing the one-way valve. As a result, this controller provides a structure where a function as a line switching valve for opening/closing the returning line of each hydraulic chamber is united to a hydraulic control valve (spool valve) controlling the hydraulic pressure supplied to each hydraulic chamber. Further, a control current value of the hydraulic control valve is controlled to control the hydraulic pressure supplied to each hydraulic chamber and at the same time, to control the switching in opening/closing of the returning line of each hydraulic chamber. Hereby, when the hydraulic pressure in each hydraulic chamber is required to be released, this controller is adapted to quickly release the hydraulic pressure through the returning line by opening the returning line of the corresponding hydraulic chamber.
Since an operating characteristic of the variable valve timing adjusting mechanism or the hydraulic control valve, however, has manufacturing variations, it is difficult to accurately perform both of the hydraulic control of each hydraulic chamber and the switching control of the returning line simultaneously by using one hydraulic control valve to which a function of the line switching valve is united. In addition, it is unavoidable that variations in a response characteristic of the vane rotor (relation between a control current value of the hydraulic control valve and a response speed of the vane rotor) occur. The variations of this response characteristic are the cause of reducing the effect (effect of an improvement in a response characteristic of an advance operation in a low hydraulic region) obtained by the one-way valve.